The present invention relates to cathodes for use in electrolytic cells operated at elevated temperatures. More specifically, the invention relates to an improved construction for cathodes comprising brittle refractory hard metal (RHM) elements seated in a carbonaceous base.
Cathodes comprising RHM elements are described in Foster et al U.S. Pat. No. 4,071,420, issued Jan. 31, 1978. In a preferred embodiment, the cathodes of the Foster et al patent are hollow sintered bodies of TiB.sub.2 extending upwardly from a floor of an electrolytic cell, through a metal pad, and toward an anode. Cathodes of the type described in the Foster et al patent are highly effective for production of aluminum after they are heated to an elevated operating temperature. However, it has been found that the hollow sintered TiB.sub.2 portions of such cathodes are brittle and subject to cracks and breakage if not protected from thermal shock, especially in the initial heat-up or start-up stages of cell operation.
A method and apparatus for protecting RHM elements from thermal shock during cell start-up are disclosed in Wiltzius U.S. patent application Ser. No. 92,441, filed Nov. 8, 1979. The three shock protecting components in a preferred form of the Wiltzius invention are plugs of CuAl alloy inside the hollow RHM elements; sleeves of insulating material extending around protruding portions of the RHM elements; and stainless steel heat dispersing jackets located externally of the insulating sleeves. The RHM elements are cemented into depressions in a carbon block. A suitable cement for attaching the RHM elements to the carbon block comprises carbon particles dispersed in a resin.
Another pertinent cell construction is shown in Steiger et al U.S. Pat. No. 4,181,583, issued Jan. 1, 1980. Steiger et al disclose several cylindrical RHM articles embedded in a carbon floor. For the purpose of insuring good electrical contact between the RHM articles and the floor, either graphite powder or graphite cement is placed between the articles and the floor.
It is a principal object of the present invention to provide an electrically conductive fibrous layer between an RHM body and a carbon base in an electrolytic cell cathode wherein the layer mechanically cushions differential thermal expansion and contraction of the body and base, thereby improving protection from thermal shock.
A related object of the present invention is to compensate for variations in sizing of RHM bodies embedded in recessed seats formed in a carbon base of an electrolytic cell.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification, considered in conjunction with the drawings.